1) Field of the Invention
The present invention relates to gas turbine engine blades and, more particularly, to the arrangement of securing the gas turbine blades to a rotating disk.
2) Description of the Prior Art
In gas turbine engines, blades are attached to disks with dovetail or firtree attachments. A section through a prior art dovetail attachment of a base of a turbine blade 3 attached to a portion of a disk 5 is shown in FIG. 1. An airfoil (not shown) is positioned above the attachment. A section through a prior art firtree attachment is shown in FIG. 2. In the first type of attachment, a blade airfoil above the line at 7 is restrained from releasing radially by a single pair of surfaces 9 and 11 whereon it makes contact with the disk 5 at surfaces 15 and 13, respectively. As a result, dovetail attachments are sometimes termed single-tooth attachments. FIG. 2 shows a prior art firtree attachment of a base of a turbine blade 23 attached to a portion of a disk 25. The attachment includes multiple pairs of contacting surfaces 27 and 29 that contact multiple pairs of disk surfaces 31 and 33, respectively. As a result, firtree attachments are sometimes called multi-tooth attachments.
During operation, the stress fields induced by contact on surfaces 9, 11 and 13, 15; and 27, 29 and 31, 33, respectively, can fluctuate in magnitude and lead to fatigue failures in blades or disks. The costs associated with these failures are on the order of millions of dollars per year. Consequently, reducing this type of failure is highly desirable both from a safety and from an economic point of view. Hence, an object of the present invention is to provide blade attachments which offer improved resistance to this type of failure.
Focusing on dovetail attachments, FIG. 1 shows a central section through the base of the turbine blade 3 and the segment of the disk 5 that affects its attachment. As a result of high angular velocities (.omega.) that can be involved, large radial forces F can be generated. In the attachment of FIG. 1, the force F is balanced by contact forces on two flats, C.sub.1 C.sub.1 ' and C.sub.2 C.sub.2 '. In order to keep the whole arrangement as compact as possible, the lengths of these flats are limited relative to the other dimensions of the blade. However, by making them as long as possible, the nominal normal compressive stress, .sigma..sub.N, on the contacting surfaces can be kept as low as possible for a given F. A consequence of keeping .sigma..sub.N down in this manner is the use of small radii at the edges of contact, for example, at C.sub.1 on the disk and C.sub.1 ' on the blade (labeled as r, r' in the close-up). This is also true for the out-of-plane direction as shown in FIG. 3 for a central section perpendicular to the section in FIG. 1. Small radii r", r'" present for this section occur at the edges of contact, that is near C.sub.3 and C.sub.3 '. For all of these small radii, contact is still conforming and stresses nonsingular. However, the actual contact stress, .sigma..sub.c, can have high gradients near the edges of contact. These high gradients lead to high peak .sigma..sub.c values. When slipping occurs with friction present, these high .sigma..sub.c in turn lead to large hoop stresses which are tensile in the blade at C.sub.1 and the disk at C.sub.1 '. These tensile stresses can then open up cracks in the blade at C.sub.1 and the disk at C.sub.1 '. With time and repeated loading these cracks can grow and ultimately give rise to failures of the attachment. Thus, reducing these tensile hoop stresses at the edges of contact can be expected to alleviate the problem of attachment failure. Hence, an object of the present invention is to reduce the tensile hoop stresses at the edges of contact in blade attachments.
Because the tensile hoop stresses are largely caused by frictional shear stresses in the contact regions, one arrangement for reducing the hoop stresses is to lower the coefficient of friction for the contacting surfaces. To this end, one practice used in the gas turbine industry is to introduce a layer of intervening material between the contacting surfaces. The material is chosen so as to facilitate slip between the blade and the disk and thereby reduce friction. It is believed that problems of attachment failure persist in the industry today even with the introduction of such intervening layers.
U.S. Pat. No. 5,110,262, which is incorporated by reference, shows an arrangement of reducing stresses at the edges of contact. This arrangement consists of making one of the in-plane contact surfaces barreled (see FIG. 3 of U.S. Pat. No. 5,110,262). This barreling reduces the peak contact stress in this plane, thus attendant shear stresses and hoop stresses. However, the height of the barreling is sufficiently large that contact with elastic stresses extends over less than half of the length of the flats (e.g., FIG. 3 of U.S. Pat. No. 5,110,262, which shows an elastic contact extent which is less than one quarter of the flats). As a result, .sigma..sub.N is increased by this arrangement. This leads to plastic flow and a redistribution of the contact stress over a larger portion of the flats. This elasto-plastic stress distribution has higher contact stresses near the edges of contact than a purely elastic or Hertzian distribution. Moreover, there is no reduction of the peak stresses near the edges of contact in the out-of-plane direction. Thus, the reduction in peak stresses near all the edges of contact afforded by the means in U.S. Pat. No. 5,110,262 is limited.
U.S. Pat. No. 5,141,401, which is incorporated by herein reference, teaches reducing peak stresses near the edge of contact in blade attachment as a way of alleviating fatigue failure. The arrangement disclosed by U.S. Pat. No. 5,141,401 to affect this end is to undercut the disk near C.sub.1 ' in FIG. 1. This patent discloses a demonstration of reduced stresses at this location as a result of such undercutting. However, if contact occurs at the break point where the undercut is initiated, stresses can be expected to be higher than without undercutting. Moreover, no arrangement is put forward for reducing peak stresses in the blade at the edge of contact near C.sub.1, nor are any arrangements put forward for reducing such stresses in the out-of-plane direction. Thus, the reduction in peak stresses near all the edges of contact afforded by the means of U.S. Pat. No. 5,141,401 are limited.